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AGU: Journal of Geophysical Research, Oceans

 

Keywords

  • methane paradox
  • sinking particle
  • stable carbon isotope of methane

Index Terms

  • Geochemistry: Marine geochemistry
  • Oceanography: Biological and Chemical: Stable isotopes
  • Biogeosciences: Marine systems
  • Oceanography: Biological and Chemical: Microbiology and microbial ecology
  • Oceanography: Biological and Chemical: Zooplankton
Abstract
Cited By (7)
 

Abstract

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 113, C03012, 7 PP., 2008
doi:10.1029/2007JC004217

Carbon isotopic characterization for the origin of excess methane in subsurface seawater

Motoki Sasakawa

Division of Earth and Planetary Sciences, Graduate School of Science, Hokkaido University, Sapporo, Japan

Japan Society for the Promotion of Science, Tokyo, Japan

Urumu Tsunogai

Division of Earth and Planetary Sciences, Graduate School of Science, Hokkaido University, Sapporo, Japan

Sohiko Kameyama

Division of Earth and Planetary Sciences, Graduate School of Science, Hokkaido University, Sapporo, Japan

Japan Society for the Promotion of Science, Tokyo, Japan

Fumiko Nakagawa

Division of Earth and Planetary Sciences, Graduate School of Science, Hokkaido University, Sapporo, Japan

Yukihiro Nojiri

National Institute for Environmental Studies, Ibaraki, Japan

Atsushi Tsuda

Ocean Research Institute, University of Tokyo, Tokyo, Japan

We collected samples of seawater, zooplankton, and sinking particles in the northwestern North Pacific to determine the source of excess CH4 over the saturation value in equilibrium with the atmospheric CH4 in the oxygenated open ocean, using stable carbon isotope as a tracer. We found that subsurface (∼100 m depth) seawater is supersaturated (up to 12%) with 13C-enriched CH4 (up to −33.1‰) relative to surface seawater in equilibrium with the atmosphere (−47‰), suggesting that in situ addition of 13C-enriched CH4 must be responsible for CH4 enrichment at depth. The δ 13C of CH4 emitted from sinking particles (from −36.7 ± 1.2‰ to +5.9 ± 7.5‰) is within the range of that of excess CH4 in seawater, suggesting that the major source of subsurface excess CH4 is sinking particles. The unusually 13C-enriched δ 13C composition of CH4 emitted from sinking particles suggests that active microbial CH4 oxidation occurs within the oxic/anoxic boundary of these particles. On the basis of the Rayleigh equation, we estimated that at least 62% of CH4 produced within the anoxic center of sinking particles is oxidized within 100 m of the surface.

Received 14 March 2007; accepted 27 October 2007; published 11 March 2008.

Citation: Sasakawa, M., U. Tsunogai, S. Kameyama, F. Nakagawa, Y. Nojiri, and A. Tsuda (2008), Carbon isotopic characterization for the origin of excess methane in subsurface seawater, J. Geophys. Res., 113, C03012, doi:10.1029/2007JC004217.

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